This application is the national phase of international application PCT/FI98/00123 filed Feb. 11, 1998 which designated the U.S.
The invention relates to a handover method in a mobile communication system in which error correction of a radio signal can be arranged with different protection levels. The method comprises measuring the signal level and/or quality of base stations in a mobile station, measuring the signal quality and/or level of the mobile station at the base station, comparing the measurement results thus obtained and other variables of the connection with handover criteria, and performing a handover from the source cell to the target cell when the handover criteria are met.
The invention also relates to a mobile communication system for performing handover.
In cellular mobile communication systems, a radio coverage area is implemented with a plurality of slightly overlapping radio cells. When a mobile station moves from one cell to another, handover to a new radio cell is performed on the basis of predetermined handover criteria. The aim is to perform handover in a manner that disturbs an ongoing call as little as possible. Handover is normally performed on the basis of radio path criteria, but it may also be performed for other reasons, e.g. to divide the load, or to reduce transmission powers. Handover may also be performed inside a cell from one traffic channel to another.
FIG. 1 of the appended drawings shows a simplified block diagram of the pan-European GSM mobile communication system. A mobile station MS is connected over a radio path to a base transceiver station BTS, in the case of FIG. 1 to BTS1. A base station system BSS consists of a base station controller BSC and base stations BTS controlled by the BSC. A plurality of base station controllers BSC usually operate under the control of a mobile services switching centre MSC. An MSC communicates with other MSCs and, through a gateway mobile services switching centre GMSC, with a public switched telephone network. The operation of the entire system is controlled by an operation and maintenance centre OMC. The subscriber data of a mobile station MS are stored permanently in a home location register HLR of the system and temporarily in the visitor location register VLR in the area of which the MS is located at a given moment.
A mobile station MS and the serving base station BTS1 continuously measure the signal level and quality of the radio connection for example to determine the need for handover. The MS measures the signals of the serving base station BTS1 and the base stations BTS that are closest to its location area for instance to select a suitable target cell for handover. In the GSM mobile communication network, for example, an MS may simultaneously measure the signal level of both the serving base station and up to 32 other base stations. Via the serving base station BTS1, the MS is informed of the neighbouring cells it should measure. The measurement results of each cell are identified on the basis of the combination of a base station identity code BSIC and the frequency of the broadcast control channel BCCH. The base station BTS measures the signal level and quality of ongoing radio connections at the base station.
The mobile station MS sends the measuring results regularly as a report message through the serving base station BTS1 to the base station controller BSC. A report message contains the measurement results of the serving base station and up to six best neighbouring base stations. Handover from a serving cell to a neighbouring cell or to another channel of the serving cell may take place, for example, when the measurement results of the mobile station/base station indicate a low signal level and/or quality of the traffic channel of the current serving cell and a higher signal level is achieved in the neighbouring cell or a better signal quality can be achieved with another channel, or when a neighbouring cell/another channel allows communication at lower transmission powers. The signal level and/or load of the target cell, for example, affect the choice of the handover target cell. Generally used handover criteria include the signal level and quality of the radio connection, the signal levels of the source cell and the target cell, the signal quality of the source cell and the transmission power required of and allowed for a mobile station in the target cell. Handover from the source cell to the target cell is performed when the handover criteria set by an operator are met. In order for the stability of the mobile communication network to be ensured, the measurement results and parameters used in handover are averaged over a given period of time. The averaging process makes handover less sensitive to measurement results distorted by instantaneous interference or fading.
The base station controller BSC makes the decisions relating to handover. If the target cell is controlled by another BSC, the handover may be performed under the control of the MSC. Another possibility is that handover decisions are always made in a centralized manner in the MSC. If necessary, the BSC gives a handover command to the MS through the BTS.
In a mobile communication system implemented by code division multiple access (CDMA) technology, handover performed in the manner described above is called hard handover. In addition, CDMA systems can use so-called soft handover, in which a mobile station, during a handover, may be simultaneously connected to the network through several base stations. When one of these base stations proves to be better than the others on the basis of its signal, the connections of the mobile station with the other base stations are released, and the call is continued only through the best base station. Soft handover prevents recurring handovers between base stations when a mobile station is located on the periphery of cells.
Transmission errors which deteriorate the quality of a transmitted signal occur on the transmission path when speech or data is transmitted in a digital telecommunication system. Transmission errors occur on the transmission path when a signal is disturbed, for example, on account of multipath propagation, an interfering signal or high background noise level. Error correction of a digital signal to be transmitted, e.g. channel coding and/or retransmission, is used for improving the quality of the transmission and the tolerance of transmission errors. In channel coding, repetition is added to the original bit string of encoded speech or data by error correcting bits calculated from the original signal. In the receiver, the channel coding is decoded in a channel decoder, whereby the signal errors that have occurred during the transmission can be detected or even corrected by means of the correcting bits. Retransmission is used for correcting transmission errors either independently or, for example, in addition to channel coding: the errors in a channel coded transmission are corrected by retransmission of distorted frames. When the quality of the connection deteriorates, the number of erroneous and lost frames grows, and thus also the number of retransmissions grows.
Channel coding increases the number of bits to be transmitted. In the GSM mobile communication system, for example, error correcting bits with a transmission rate of 9.8 kbit/s are added to a full-rate 13 kbit/s speech signal, whereby the total transmission rate is 22.8 kbit/s. The level of the protection provided by channel coding is arranged according to the need. If a large number of data are to be transmitted fast, the amount of channel coding is reduced to allow more payload to be transmitted on the transmission channel. Channel coding may be arranged either to both detect errors occurred during the transmission and correct them or to merely detect them. In the GSM system, bits to be transmitted are divided according to their importance into different classes, in which channel coding is provided at a predetermined level. The different elements of the mobile communication system may limit the selection and, implementation of the channel coding provided for a connection. A mobile station may, for instance, support only certain channel codings. Furthermore, the protection level of the provided channel coding depends on the capability of the base station and other network elements to employ different channel codings.
A problem in the handovers of the prior art is that as network planning is generally made for channels using normal error correction, it is not always possible to determine an appropriate handover target cell with the handover procedure, when the quality of a connection performed with weak error correction indicates the need for handover. In addition, a problem in handovers based only on the level of base station signals is that the quality of a radio connection performed with weak error correction may deteriorate too much before handover is performed and the handover of a radio connection performed with efficient error correction is performed too soon.
An object of the present invention is to provide an optimal way of performing handover in an environment where error corrections of several different levels are used.
This new type of handover is achieved with a method of the invention, which is characterized by determining for a connection at least one channel change criterion depending on the error correction of the radio connection.
Another object of the invention is a mobile communication system where error correction of a radio signal can be arranged with different protection levels, said system being arranged to measure the signal level and/or quality of base stations received in a mobile station, to measure the signal quality and level of the mobile station received at a base station, to compare the measurement results thus obtained and other variables of the connection with handover criteria, and to perform a handover from the source cell to the target cell, when the handover criteria are met. According to the invention, the mobile communication system is characterized by being arranged to determine at least one handover criterion depending on the error correction of the radio connection.
The invention is based on the idea that, when a handover decision is made, the effect of error correction on the signal level required on the radio connection is taken into account.
In the handover method of the invention, the error correction of the connection, preferably channel coding, is used as a criterion for handover. In a first embodiment of the invention the handover criterion according to the invention is set in such a way that it is met with a target cell base station signal that is better than normal when a weaker error correction has to be used on the target cell channel than on the source cell channel, and with a target cell base station signal that is weaker than normal when a more efficient error correction can be used on the target cell channel than on the source cell channel. A functionality can also be added to the first embodiment of the invention, on the basis of which functionality, when error correction is the same in the target cell and the source cell, the handover criterion of the invention is set in such a way that it is met with a target cell base station signal that is better than normal when the common error correction is more efficient than normal, and with a target cell base station signal that is weaker than normal when the common error correction is weaker than normal. Handover is performed to the target cell when the handover criterion of the invention and other possible handover criteria of the prior art are met, and simultaneously the protection level of error correction of the connection in the target cell is is set to the level determined during handover. In a second embodiment of the invention a handover priority is determined for the connection on the basis of the error correction, preferably channel coding, employed on the radio connection in the source cell. A handover criterion triggers handover to different connections in the order of their priority values, when the other handover criteria of the connection are met.
An advantage of such handover is that handover can be flexibly arranged depending on error correction.
Another advantage of the handover of the invention is that unnecessary handovers are avoided, when the handover of the radio connection implemented with error correction that is more efficient than normal is performed, when it is necessary regarding the quality of the connection.
Yet another advantage of the handover of the invention is that a network planned for normal error correction can offer optimal handover even to mobile stations whose error correction is implemented at a protection level different from the normal.
A further advantage of the handover of the invention is that a change in the protection level of error correction during handover ensures that the quality of the radio connection to be handed over remains sufficiently good.
Still another advantage of the claimed handover is that it reduces the probability of a call being lost on a radio connection implemented with weaker error correction than normal.